EP0091202B1

EP0091202B1 - Improved tape timer for tape recorder/reproducer

Info

Publication number: EP0091202B1
Application number: EP83301245A
Authority: EP
Inventors: Rick L. Reagan
Original assignee: Ampex Corp
Current assignee: Ampex Systems Corp
Priority date: 1982-04-02
Filing date: 1983-03-08
Publication date: 1990-05-16
Anticipated expiration: 2003-03-08
Also published as: EP0091202A3; DE3381575D1; JPH0792993B2; EP0091202A2; US4471391A; JPS58189886A

Description

This invention relates to tape drive apparatus and, more particularly, to an improved tape timer system for indicating position of recording tape, moved by a drive mechanism, relative to a tape head.
Tape recording systems generally include provisions for indicating the longitudinal position of recording tape relative to a tape head, e.g., a magnetic record or reproduce head. These devices, typically known as tape timers, are known as such because the tape position indication is often displayed in a scale of time (hours, minutes, seconds) or, in the case of video recording systems and television signals, also frames. Prior art tape timer systems have assumed four different basic forms, namely: purely mechanical tape counters, tachometer type systems, control track type systems, and combinations of tachometer and control track systems.
The purely mechanical type counters are often found in inexpensive audio recorders. Such recorders are usually driven by a belt attached to a reel or to a tape idler, and consequently produce a non-linear relationship between the generated indication and actual tape position. A disadvantage of the mechanical type counter is, however, that there is usually a considerable amount of slip and consequently a high degree of inaccuracy associated with its use. In addition, they do not provide electrical signals related to tape travel.
The tachometer type tape system develops periodic electrical pulses related to the movement of the drive mechanism which, in turn, is related to tape movement. Typically this is done by employing a tachometer wheel coupled to rotate as the tape is transported. Tachometer sensors produce pulses as the tachometer wheel rotates and these pulses are counted to provide information as to the length of tape which has been transported past the tape head. The tachometer information is accordingly mechanically derived and its accuracy therefore depends upon maintaining a constant relationship between the number of tachometer pulses generated and the actual longitudinal length of tape transported past the magnetic head location. Tape slippage, creep, changes in tape stretching and transport tension, the fact that the tape is reproduced on different recorders than it was recorded on with differences in tension, capstan diameters and capstan surface slip coefficients, etc., cause changes in the relationship which, in turn, cause inaccuracies in the length measure.
The control track type system utilizes timing information recorded on the tape itself, which information is read from the tape as it passes the magnetic head location. Such timing information is recorded on the tape to provide information for controlling the operation of tape recorders. However, such information has been used for tape timing purposes as well. Pulses are developed from information recorded directly on the tape, which are directly related to the longitudinal spacing of recorded information on the tape. Such a system circumvents the inaccuracies of the mechanical tachometer system since such recorded information accurately reflects the length of tape transported past the mechanical head locations. This avoids inaccuracies due to stretch or shrinkage of the tape or slippage thereof.
In some circumstances, such as in video recording applications, there may be a total absence of recorded timing information on the tape for a portion of the tape length. Under such circumstances, the absence of timing information causes a consequent inability of the tape timer to function.
To counteract the problems inherent in th'e tachometer type and control track type tape timer systems, tape timer systems have been developed employing a combination of the tachometer and control track systems, as illustrated in U.S. Patent 4,037,260, entitled Tape Timer Error Corrector Circuit For Tape Recorder/ Reproducer issued July 19, 1977 in the name of Edwin S. Busby and Reginald W. Oldershaw, and U.S. Patent 4,062,048 entitled Tape Timer Apparatus and Method Employing A Phase Comparison Between Sequential Pulse Trains issued July 19, 1977 in the name of Donald William Webber. A tachometer pulsing system is used to provide consistent timing pulses related to the mechanical drive mechanism and these pulses are employed as the principal source of tape timing information. Control track pulses are also detected, if present, and are employed in a suitable fashion to adjust the tachometer pulses to correctly relate them to the length of the tape transported past the recording head. Such a system avoids timing inaccuracy due to tape slippage and changes in tape length inherent in the tachometer type systems while at the same time avoiding disfunction caused by dropout information inherent in control track timing systems.
Although the combination type systems described in the previous paragraph provide freedom from the problems mentioned, prior art systems of this type have suffered from certain limitations. Among these limitations is the inability of such systems to accommodate variations in longitudinal tape speeds, thus requiring operation at fixed longitudinal tape speeds.
In addition, such combination systems frequently suffered from spurious response to tape noise.
In addition, such prior art systems, due to the techniques employed in attempts to limit interference from spurious response to tape noise, have resulted in designs which are insensitive to control track pulses which occur during particular portions of longitudinal tape motion, and in fact rely on speed variations to correct this problem.
Furthermore, prior art systems have limited abilities to response to detected differences between tachometer and control track pulses, generally being limited to changes in fixed amounts. Consequently, they have been unable to dynamically adjust the amount of correction employed in response to varying amounts of differences detected between tachometer and control track pulses.
In one aspect the invention provides apparatus for indicating the longitudinal position of a recording tape driven by a drive mechanism comprising means for producing first pulses in response to and denoting movement of the tape; a counter for counting said first pulses; control means for controlling the counter to count in a succession of similar sequences; means for producing an indication of the said longitudinal position in accordance with the number of sequences executed by the counter; and means for reproducing signals representing intervals recorded on the tape to provide second pulses; characterised in that in response to the occurrence of a said second pulse the control means compares the current count of the counter with a predetermined count and on the detection of any discrepancy resets the counter within the current sequence so that the sequence is lengthened or shortened to reduce phase error between the said second pulses and the counting sequences.
In another aspect the invention provides a method of indicating the longitudinal position of a tape driven by a drive mechanism, comprising developing first pulses by means of a tachometer driven by a rotary member engaging the tape, counting the first pulses in a succession of similar counting sequences, providing an indication, related to the number of sequences executed, of said longitudinal position, and playing back second pulses recorded in a longitudinal control track on the tape, characterised by comparing in response to a second pulse the actual count then reached in the current sequence with a predetermined count and on the detection of any discrepancy resetting the count so that the length of the sequence is lengthened or shortened to reduce phase error in the sequences relative to the second pulses.
In a preferred form of the present invention, periodic electrical first pulses are developed by a tachometer system related to movement of the tape drive mechanism which is in turn related to longitudinal tape movement relative to the tape head. Periodic electrical second pulses are developed related to longitudinal spacing of recorded information on the tape from the control signal previously recorded on the tape. A series of count sequences of dynamically varying length is produced by an electrical counter in response to pulses from the tachometer system and control signals from a phase difference detection system. Upon detection of a phase difference between the control track pulses and the pulses generated by the mechanical tachometer, the count sequence of the counter is dynamically modified, by an amount which is determined by the magnitude of the detected phase difference. Consequently the present invention can provide correction in response to a detected phase difference in amounts which are determined by the amount of phase difference detected. An indication of tape position is produced which is related to the completion of count sequences. Thus, the tape position indication is adjusted as a result of the phase shifting of the sequence count pulses to coincide with the control track pulses.
In the present invention, the amount of phase adjustment affected in response to a detected phase shift is easily variable at the time of system design. Consequently, the response of the present invention to a detected phase error can be varied depending upon the desired response characteristics of a system incorporating it.
In addition, the present invention may provide an improved technique for responding to control track pulses in the presence of noise.
The invention, as described herein, is particularly related to and advantageous in the case of video tape recording and is described, at least in portions, in that context. It will be apparent to those skilled in the art, however, that the invention is applicable to the tape timing systems used in connection with the recording of information other than video signals.

Figure 1 illustrates the basic layout of a magnetic tape transport.
Figure 2 is a block diagram which illustrates the method of operation of the present invention.
Figure 3 is a series of timing diagrams labeled 3A-3F which illustrates the timing relationships present in and the operation of the present invention.
Figure 4 is a schematic diagram of the implementation of the preferred embodiment of the present invention.
Figure 5 is a flow chart which illustrates the operation of the present invention in response to certain signals.

Referring now to Figure 1, a tape transport for longitudinal recording is illustrated for simplicity, although an arrangement for helical or other recording scheme may be used in the same manner. Magnetic tape 10 stored on reels 12 is moved past magnetic heads 14 by a capstan 18 associated with a pinch roller 16. Magnetic tape' 10 partially wraps around hub 20. Consequently when magnetic tape 10 is moving longitudinally, hub 20 rotates in a direction determined by the direction of motion of magnetic tape 10.
The shaft of the hub 20 is coupled to a tachometer (not shown) which produces electrical pulses in response to rotation of the hub. The frequency of pulses so produced for a given longitudinal tape speed is a matter of choice at the time of design of the tachometer. For a given tachometer design, the total number of pulses generated between receipt of sequential control track pulses is a known quantity which, in the absence of tape slippage or creep, etc., is a constant. Consequently by counting the number of tachometer pulses received between sequential control track pulses, the presence of tape slippage, creep, etc., can be detected as a change in phase between control and tachometer pulses.
The general method of operation of the preferred embodiment of the present invention can be understood by reference to Figure 2. Counter 32 is an electronic counter, which is arranged to decrement by a count of one upon receipt of each tachometer pulse 30. Control means 36 functions to drive a tape count display 46 which indicates tape position in terms of hours, minutes, seconds, and frames, and to control the operation of counter 32 in response to control track pulse signal 42, tape direction signal 44 and 'count equals zero' signal 40.
Control means 36 initially places a value in counter 32 by means of a load signal 34 which represents the total number of tachometer pulses expected to be received between sequential control track pulses 42. This number will hereafter be referred to as the count modulus. Thereafter, on the occurrence of each tachometer pulse 30, the contents of counter 32 are decremented by one. When the value present in counter 32 reaches a datum (preferably zero), control means 36 is advised of same by 'count equals zero' signal 40. Thereafter, control means 36 again loads counter 32 with the number representative of the total number of tachometer pulses 30 expected to be received between sequential control track pulses 42, and accordingly updates the tape count display 46 to indicate the corresponding longitudinal tape displacement in the appropriate manner, depending upon the direction of tape motion, as supplied by tape direction signal 44.
In the foregoing discussion the occurrence of control track pulses 42 was not considered. Control track pulses 42 may occur at any point during the decrementing process taking place in counter 32 between the initial loading of a starting number by load signal 34 and the occurrence of zero in the contents of counter 32 reaching zero. The point in time at which control track pulse 42 will normally be received is a matter of choice at the time of system design. In the preferred embodiment, control track pulses 42 are received when the count present in counter 32 has decremented to half of its initial value. Consequently, upon receipt of a control track pulse 42, control means 36 will determine the present value of the number present in counter 32 by 'current count' signal 38, and compare this value against the known correct value. A non-zero different between the current count present in counter 32 and the known correct count indicates the presence of a phase difference between the control track pulses 42 and the tachometer pulses 30. The magnitude of difference in counts is indicative of the magnitude of said phase difference. The sign of said difference in counts indicates the sense of said phase difference.
When a phase difference has been detected, control means 36 will thereafter adjust for same by placing a new value in counter 32 by a load signal which reflects the desired amount of correction. Thereafter, counter 32 will continue to decrement upon the receipt of each tachometer pulse 30 until the contents of counter 32 reach zero. At this point, control means 36 is advised of said status by the 'count equals zero' signal 40, and the process repeats.
Control means 36 up-dates the tape count and consequently the tape count display 46 each time the contents of counter 32 reach zero. Consequently by adjusting the value present in counter 32 upon the occurrence of each control track pulse 42 in response to a detected phase error, the adjustment for a detected phase error is reflected in the value in tape counter display 46.
As the occurrence of a control track pulse directly controls the detection of the presence of a phase shift, it is clear that spurious signals present on this signal line would have a direct impact on the accuracy of the present system. In the present invention improved noise immunity is achieved by control means 36 disregarding any subsequently received pulses on the control track pulse input after receipt of an initial control track pulse 42 until the contents of counter 32 reach zero. It is only thereafter that the present invention will again start "looking" for the presence of a control track pulse 42. In this manner, the present invention is insensitive to spurious signals which may be present on the input for control track pulses between the occurrence of a first control track pulse and the subsequent occurrence of zero for the contents of counter 32.
An example will serve to illustrate the operation as heretofore described. It is of course clear that the following example is for illustrative purposes only, and in no way is intended to limit the present invention.
For the purpose of the following example, it will be assumed that the count modulus is twelve, i.e., twelve tachometer pulses are received between successive control track pulses.
Figure 3 illustrates the timing relationships among the various signals. Figure 3A illustrates the tachometer pulses 30 and the corresponding contents of counter 32. Figure 3B illustrates the 'count equals zero' signal 40. Figure 3C illustrates load signal 34, and the corresponding numerical value loaded into counter 32. Figure 3D illustrates control track pulses 42. Figures 3E and 3F illustrate conditions internal to control means 36, i.e. the signal which up-dates the tape counter and the period of time during which control means 36 disregards control track pulses.
Counter 32 (Figure 1) is initially loaded with the count modulus twelve, as indicated by pulse 50 in Figure 3C. Thereafter, as shown in Figure 3A, the contents of counter 32 are sequentially decremented upon receipt of each tachometer pulse. Upon receipt of control track pulse 52, Figure 3D, control means 36 determines the current contents of counter 32 by the 'current count' signal 38 (Figure 2). In the present example this is the value of six, as indicated by pulse 54 in Figure 3A. Control means 36 determines the difference between the current contents of counter 32 and the expected count (six). If the difference is zero, a zero phase shift between control track pulses and tachometer pulses is indicated. As a zero phase shift has been determined, it is not necessary to adjust the counting sequence of counter 32. Control means 36 thereafter disregards any subsequent pulses from control track pulses 42 as illustrated by pulse 56 in Figure 3F.
Counter 32 continues to decrement in response to subsequently received tachometer pulses 30. When the contents of counter 32 equal zero, as indicated by pulse 58 in Figure 3B, control means 36 does several things. First, control means 36 updates the count present in the tape count display. Depending upon the direction of tape motion this will result in the current value being either incremented or decremented. Next, control means 36 will recognize any subsequently received control track pulses 42. Finally, control means 36 will load counter 32 with the count modulus twelve, as indicated by pulse 60 in Figure 3C. Thereafter, the contents of counter 32 will be decremented by one upon receipt of each sequential tachometer pulse, as indicated in Figure 3A, beginning with pulse 62.
If there has been any type of slippage, the control track pulse will be displaced from the selected tachometer pulse. This is illustrated by control track pulse 62 (Figure 3D), which occurs when the count in counter 32 equals eight, as illustrated by pulse 64 in Figure 3A. Control means 36, upon sampling the current contents of counter 32, subtracts the value so determined (eight) from the expected value (six) and upon determination of a non-zero difference, will load counter 32 with a new value (six) to effect an adjustment of the phase relation so detected, as illustrated by pulse 63 in Figure 3C. The desired value loaded into counter 32 in response to a detected phase error is a parameter determined at the time of system design, and can be so chosen as to vary depending upon the amount of phase error detected, e.g., if the count in counter 32 was seven or eight, counter 32 could be reloaded with the value of six, but if the phase error was greater, as would be indicated by the contents of counter 32 being, for example, eleven, counter 32 could be reloaded with the number nine. In this manner, the response of the present invention to a detected phase error can be adjusted to that desired for the particular system the present invention may be used in.
Thereafter, the contents of counter 32 are sequentially decremented upon receipt of tachometer pulses 30, and the process continues as above described.
Consequently it is observed that the present invention, upon the detection of a phase error between control track pulses 42 and tachometer pulses 30, responds thereto with a corresponding adjustment in phase which dynamically varies in magnitude depending upon the amount of phase error detected.
Furthermore, it is observed that disregarding subsequent control track pulses upon the receipt of a first control track pulse until the contents of the counter decrement to zero, results in improved immunity from noise results.
While the foregoing has described the present invention by means of a general functional description and a specific operational example, the following will describe the present invention in terms of the preferred embodiment, as illustrated in Figure 4.
Microprocessor 70 communicates with the various devices associated therewith by address bus 72, data bus 74, and the following control signals: IRQ' 76, R/W' 78 and E 80. IRQ' 76.is the signal used by devices to notify microprocessor 70 that they require attention; in particular, IRQ' 76 is the interrupt signal for microprocessor 70. R/ W' 78 is the signal used by the microprocessor 70 to indicate to external devices whether it is performing a read or write operation. E 80 is a timing signal. Microprocessor 70 may be constituted by a Motorola 6802 microprocessor.
Memory 82 serves to store the instructions and data associated with microprocessor 70, and is preferably implemented using erasable head only memories model 2732 produced by Intel Corporation, and random access memories model 4118A produced by Mostek.
Address decoder 84 functions to decode specific addresses present on address bus 72, generating a unique signal for each individual address decoded which is of interest: IRQ request 112, IRQ acknowledge 114, counter select 116, and a plurality of display select signals 118. Address decoder 84 is preferably implemented with a Texas Instruments 74LS138.
Counter 86 functions to decrement by a count of one upon receipt of each tachometer pulse 30. Counter 86 is loaded with an initial value from data bus 74 upon the selection of counter 86 by the address associated therewith being decoded by address decoder 84 to generate counter select signal 116. Concurrent with the generation of counter select signal 116, the value present on data bus 74 is transferred to counter 86 upon the occurrence of W' signal 90, which is generated by microprocessor 70 upon the execution of instructions appropriate to perform a write operation, thereby generating R/W' signal 78 and E signal 80.
In the preferred embodiment, counter 86 is a decrementing counter, hence a change in tape direction will not result in a change of the direction of count by counter 86. Consequently, the necessary change in counting sequence is performed by reloading counter 86 with the difference between the count modulus and the current value present in counter 86 upon a change in tape direction. By so doing, the count sequence so generated by counter 86 will be the desired one, generating the necessary'count equals zero' indication at the desired time.
The contents of counter 86 are read by microprocessor 70 upon the selection of counter 86 by the address associated therewith being decoded by address decoder 84 to generate counter select 116. The contents of counter 86 are thereafter transferred to data bus 74 upon the generation of R' signal 92 by microprocessor 70. This is accomplished by microprocessor 70 upon the execution of a read instruction which results in the generation of R/W' signal 78 and E signal 80. Counter 86 is preferably implemented by use of an Intel 8253.
Microprocessor 70 performs the necessary tape counting incrementing or decrementing, as the case may be depending upon the direction of tape motion, each time the contents of counter 86 reach zero. The resultant count is supplied to the appropriate latch 94 as defined by the individual addresses associated with the individual digits, as decoded by address decoder 84, generating display signal 118, and subsequently displayed by display 46.
The occurrence of a change in tape direction, a control track pulse or the contents of counter 86 reaching zero are communicated to microprocessor 70 through the use of interrupts. The occurrence of each of the above events results in a flip-flop being set: the occurrence of a change in tape direction sets flip-flop 98, the occurrence of a control track pulse set flip-flop 100, and the occurrence of the contents of counter 86 reaching zero sets flip-flop 102. The flip-flops associated with these events are preferably implemented using a Texas Instrument 74LS74.
The outputs of flip- flops 98, 100, and 102 are supplied to priority encoder 104. Priority encoder 104 functions to generate a binary code which uniquely defines which of the currently active signals supplied to it has been assigned the highest priority, as defined by the manner in which the signals are connected to the inputs to priority encoder 104. The interrupts are arranged with tachometer pulse 30 having the highest priority, followed by control track pulse 42, and the tape direction signal having the lowest priority of the three interrupts. Upon the occurrence of any signal or combination thereof to the inputs of priority encoder 104, priority encoder 104 generates IRQ' signal 106 which serves to notify microprocessor 70 of the presence of an interrupt. Priority encoder 104 is preferably implemented with a Texas Instrument 74LS148.
Upon the occurrence of an interrupt by the presence of IRQ' signal 106, microprocessor 70 will place the appropriate address on address bus 72 which is thereafter decoded by address decoder 84 to generate the appropriate signal on IRQ' request signal 112 to enable the transfer of the output of priority encoder 104 by tri-state driver 108 onto data bus 74. In this manner, the source of the interrupt can be determined. Tri-state drivers 108 is preferably constituted by a Texas Instruments Type 74LS541.
After microprocessor 70 has responded to the interrupt, the appropriate interrupt flip- flop 98, 100 or 102 is thereafter selectively cleared by microprocessor 70 placing the appropriate binary word on data bus 74 and thereafter placing the corresponding address on address bus 72 which, when decoded by address decoder 84, results in the generation of IRQ acknowledge signal 114 which enables latch 100. Latch 110 functions to hold the binary word present on data bus 74, and thereafter clears the appropriate flip- flop 98, 100, or 102 as defined by the binary word stored therein.
As latch 110 will hold the "present" inputs to flip- flops 98, 100 and 102; in the present state as long as the associated binary word is present in latch 110, the duration of time during which subsequently occurring interrupts will be ignored can be easily controlled, i.e., when it is desired to recognize subsequently occurring interrupts, the input to the associated flip-flop will be allowed to return to their alternate state by the presence of the appropriate binary word being stored in latch 110 by microprocessor 70.
Furthermore, as each of the several flip-flops can be individually cleared by this arrangement, any outstanding interrupts which were of a lower priority and consequently not identified by priority encoder 104 will remain set, and result in the continual presence of IRQ' signal 106. The process will thereafter repeat until all interrupts have been communicated to microprocessor 70.
The operation of the preferred embodiment of the present invention can best be understood with reference to Figure 4 and Figure 5 which illustrates in a flow chart format the operation of the present invention in response to'count equals zero', 'control track' and 'tape direction' interrupts.
As the preferred embodiment was implemented using an interrupt driven microprocessor design, microprocessor 70 responds to control track pulse signals, 'count equals zero' signals or tape direction signals when they occur. Figure 5 illustrates the response of microprocessor 70 in a flow chart format to the three individual interrupts. Response to a 'count equals zero' is shown in Figure 5A, response to a control track pulse is shown in Figure 5B, and response to a tape direction pulse is shown in Figure 5C.
It will first be assumed that counter 86 has been previously loaded with the count modulus, which in the preferred embodiment is twelve, and has thereafter decremented upon receipt of tachometer pulse 30 to the point where the contents thereof have reached zero. At this point, 'count equals zero' signal 40 will set flip-flop 102 which results in priority encoder 104 generating IRQ' signal 106 to microprocessor 70 and simultaneously encoding said interrupt. Referring to Figure 5A, microprocessor 70 will respond by thereafter allowing control track pulse 42 to interrupt and subsequently enabling flip-flop 100 by storing the appropriate binary word in latch 110. Thereafter, microprocessor 70 will up-date the tape count and output the current value to tape count display 46. Next microprocessor 70 will re-load counter 86 with the count modulus, and then clear flip-flop 102 which clears the'count equals zero' interrupt as above described. Microprocessor 70 will thereafter return to the main program.
Upon the occurrence of a control track pulse 42, flip-flop 100 will be set. This results in priority encoder 104 generating an IRQ' signal 106 and simultaneously encoding said interrupt. Referring to Figure 5B, microprocessor 70 will respond by noting that it will thereafter disregard further control track pulses 42 and store the appropriate binary word in latch 110 to result in flip-flop 100 remaining continuously pre-set. Next, microprocessor 70 will determine the contents of counter 86. Thereafter the contents of counter 82 will be subtracted from the expected count. The presence of a non-zero difference will result in microprocessor 70 determining the new count to be loaded into counter 86 based upon the difference from values specified at the time of system design, and thereafter loading these values into counter 86.
In the preferred embodiment, the number loaded into counter 32 (Figure 1) in response to a detected phase difference is determined by algebraically adding the numeric results of the subtraction process used to determine the presence of a phase shift to the current contents of counter 32 if the absolute value of said difference is equal to or less than two. If the absolute value of said difference is greater than two, the number +3 or -3 is algebraically added to the current contents of counter 32 depending upon the results of said subtraction process being positive or negative, respectively. Microprocessor 70 will thereafter return to the main program. If the difference from the above described subtraction was zero, microprocessor 70 would respond by returning to the main program without loading a new count into the counter.
Microprocessor 70 is informed of a change in the direction of tape travel by the occurrence of a pulse on tape direction signal 44, which results in flip-flop 98 being set. Thereafter, priority encoder 104 generates an IRQ' signal 76 to microprocessor 70 which results in microprocessor 70 first changing the direction of the count sequence in the tape counter (Figure 5C). Next, microprocessor 70 determines the current contents of counter 86, and subtracts the contents thereof from the count modulus. The results of the subtraction are thereafter loaded back into counter 86. Thereafter, microprocessor 70 clears the tape direction interrupt by clearing flip-flop 98 by the process of loading the appropriate binary word into latch 110, and thereafter return to the main program.

Claims

1. Apparatus for indicating the longitudinal position of a recording tape (10) driven by a drive mechanism (16, 18), comprising means for producing first pulses (30) in response to and denoting movement of the tape; a counter (86) for counting said first pulses; control means (70) for controlling the counter to count in a succession of similar sequences ("11"-"0"); means (54,46) for producing an indication of the said longitudinal position in accordance with the number of sequences executed by the counter (86); and means (14) for reproducing signals representing intervals recorded on the tape to provide second pulses (42); characterised in that in response to the occurrence of a said second pulse (42) the control means compares the current count of the counter (86) with a predetermined count ("6") and on the detection of any discrepancy resets the counter (86) within the current sequence so that the sequence is lengthened or shortened to reduce phase error between the said second pulses (42) and the counting sequences.

2. Apparatus according to claim 1 in which the means for providing the first pulses (30) comprises a tachometer driven by a rotary member (20) engaging the tape.

3. Apparatus according to claim 1 or claim 2 in which the said signals that represent said intervals are constituted by a longitudinal control track on the tape.

4. Apparatus according to any foregoing claim, in which the control means (70) inhibits a reset of the counter (86) more than once during any one counting sequence.

5. Apparatus according to any foregoing claim in which the control means (70) resets the counter (86) to the said predetermined count on the detection of the said discrepancy.

6. A method of indicating the longitudinal position of a tape (10) driven by a drive mechanism (16, 18), comprising developing first pulses by means of a tachometer driven by a rotary member (20) engaging the tape, counting the first pulses (30) in a succession of similar counting sequences ("11'⁼"0"), providing an indication, related to the number of sequences executed, of the said longitudinal position, and playing back second pulses (42) recorded in a longitudinal control track on the tape, characterised by comparing in response to a second pulse the actual count then reached in the current sequence with a predetermined count ("6") and on the detection of any discrepancy resetting the count so that the length of the sequence is lengthened or shortened to reduce phase error in the sequences relative to the second pulses.

7. A method according to claim 6 in which the indication of tape position is updated in response to the completion of each counting sequence.

8. A method according to claim 6 or claim 7 in which a second resetting of the count before the completion of any one sequence is prevented.